Flippers for swimming are generally produced by injection molding of thermoplastic resins which can be carried out in one step in the case in which, to lower the production costs, the fin and shoe are molded simultaneously, or in at most two successive steps in the case in which the shoe is injected onto a separately molded fin. In the latter case, it is possible to obtain flippers of higher quality since softer materials can be used for the shoe and more rigid ones for the fin. The most commonly used thermoplastic resins are those comprising mainly ethylene/styrene and butylene/styrene for the shoe, and mainly EVA (ethylene-vinyl acetate copolymer) for the fin.
With regard to special flippers such as those made for expert scuba divers, the flippers must be highly technical and comfortable. In this case, a widely used type of flipper comprises a fin in moplen of considerable length and a shoe, made of a mixture composed mainly of natural rubber, anchored to one end of the fin and to its lateral edges by means of ribs extending from it. The shoe differs in hardness in its various parts to accommodate the specific functions that each of them must accomplish: thus, its front part, in contact with the toes, the rear part and the edge delimitating the opening of the shoe are of a soft consistency, whereas the part corresponding to the back of the foot and to the fin connecting ribs is considerably more rigid. This is accomplished in the molding step, using portions of rubber of different hardnesses which, also on the basis of the experience of the operator, are adequately arranged in each part of the mold before vulcanization. With this procedure, high quality flippers, from the standpoint of both performance and comfort, are obtained, but the molding time is relatively long, and, therefore, the productivity is low. Furthermore, due to these molding modalities, limited but perceptible irregularities can occur in the distribution of the differences in the stiffness of the shoe with respect to the optimum distribution, thus involving some localized discomfort for the person wearing the flipper.
With the object of reducing the time and cost of production, the use of rubber has been replaced by that of thermoplastic resins, to realize flipper shoe and ribs by means of injection molding. The flippers that are obtained with this procedure are, however, of lower quality and are also less comfortable with respect to the above described flippers made of rubber. This is because the stiffness of the ribs which extend from the shoe along the sides of the fin affect the stiffness of the shoe itself and, therefore, the resulting stiffness is a compromise between the lesser stiffness with which the shoe should be produced to assure adequate comfort and the greater stiffness which should be used to assure adequate elasticity of the fin.
The object of the present invention is to provide a new production method which makes it possible to obtain, by means of injection molding of all its components, a flipper for swimming with technical characteristics, performance and comfort not lower than those of already known flippers made of natural rubber, but with considerably reduced production time and costs.
A further object of the present invention is to provide a flipper having a new structure to allow the control of the optimal distribution of the various degrees of stiffness of the various components thereof.